1. Field of the Invention
This invention relates generally to electrically-activated switches and, more particularly, to switches that can be activated to allow current to bypass an electrical component.
2. Description of the Related Art
In a wide variety of electrically-operated devices, it sometimes becomes necessary to bypass a defective component. For example, one cell of a multi-cell battery sometimes can fail after being placed in use. A battery cell can fail, for example, if the battery is physically damaged, if the chemicals in the battery evaporate away, or if the battery electrodes corrode. When a battery cell fails, the internal resistance of the cell typically increases, which inhibits charging of the cell and can damage the remaining cells in the battery, causing the entire battery to fail. Such a cell failure also can electrically isolate the remaining cells, again causing the entire battery to fail. In many instances, such as if the battery is located in a satellite in earth orbit, the battery cell cannot be easily replaced and therefore is preferably bypassed.
A battery cell typically is bypassed using a bypass loop that includes both charge diodes and a discharge diode, connected in parallel with the battery cell. In the case of a multi-cell battery, each cell must be fitted with such a diode bypass loop. Typically, the charge diodes include several diodes connected in series such that their forward-bias voltage is greater than the cell voltage. These charge diodes do not affect normal operation of the battery cell. However, if the cell becomes defective and its resistance increases, then, during battery charging, the cell and charge diodes constitute a current dividing network, and substantially all of the charge current will be shunted around the defective cell and through the charge diodes. The individual current rating of the charge diodes ordinarily is on the order of 10 amperes or less.
The discharge diode typically is a single diode that is reverse biased during battery charging but forward biased to conduct current during battery discharging. In this way, during battery discharging, the battery current is shunted around the defective cell and through the discharge diode. The current rating of the discharge diode ordinarily is on the order of 50 amperes or more.
Although providing a battery network with diode bypass loops of this kind effectively solves the problem of bypassing defective battery cells that are inaccessible, particularly in satellite applications, use of the diode bypass loops also carries certain disadvantages. For example, conducting current through the diodes after a battery cell failure causes a voltage drop and generates heat. The thermal design of the battery must take this into account. The generation of heat also means that the physical location of the diodes can become critical. Also, as the cell size increases, the diode size, weight, and cost can become significant. Further, the voltage drop from the diode during discharge detracts from the total voltage that the battery would otherwise supply. Additional cells therefore might be required, to achieve a desired overall battery voltage.
From the foregoing discussion, it should be apparent that there is a need for a component bypass switch that can bypass a defective component, such as a defective battery cell, and that can do so with minimal weight, cost, and thermal restrictions. The present invention satisfies this need.